How would you do this?

Now I know there are some geniuses out there
What I want to do on a membership portal is assign a number to a group of members when they register. I need this number to goto the next number after x amount of people are registered under one number:

Example
Member 1 - Group 1
Member 2 - Group 1
Member 3 - Group 1
Member 4 - Group 1
Member 5 - Group 1

Member 6 - Group 2
Member 7 - Group 2
Member 8 - Group 2

etc

Do any of you geniuses have an ideas how to implement this?